Sensor arrangement

ABSTRACT

The invention relates to planar sensor arrangements, in particular seat mat sensors, for recognition of seat occupancy in a motor vehicle, comprising several pressure sensitive sensor elements (A,S), arranged in a planar distribution, the electrical properties of which are each dependent on the local value of a measured parameter and with at least one non-rotationally symmetrical sensor element (A) on an installation-dependent fold line of the sensor arrangement, with a longest sectional line ( 12 ) through the active surface of the sensor element (A) arranged along the fold line (K).

CLAIM FOR PRIORITY

This application claims priority to International Application No.PCT/DE02/04429, which was published in the German language on Jul. 3,2003, which claims the benefit of priority to German Application No. 10160 121.2 which was filed in the German language on Dec. 7, 2001.

TECHNICAL FIELD OF THE INVENTION

The invention relates to planar sensor arrangements, in particular seatmat sensors for recognition of seat occupancy in a motor vehicle.

BACKGROUND OF THE INVENTION

Seat occupancy by motor vehicle passengers plays a major role in aplurality of technical applications in motor vehicles. This appliesespecially to vehicle occupant restraint systems, the efficientdeployment of which is very often made dependent upon the seatingposition of the vehicle occupant.

The simplest form of recognizing seat occupancy is the detection of thepresence of a vehicle occupant by, for example, touch-sensitive switchesin the vehicle seat. Other measurement systems for the recognition ofseat occupancy detect the weight of a vehicle occupant or even theirweight distribution on a vehicle seat. On the one hand, this enables thedetection of the presence of a vehicle occupant whilst, on the otherhand, their body weight or weight distribution on the vehicle seat canbe determined. Depending on the sensor arrangement and measurementmethod, these two sets of data can be measured simultaneously orindependently of each other.

Seat mat sensors are mostly used to detect the measured variable, inparticular weight, comprising a plurality of pressure-sensitive sensorelements which are arranged on the seat face distributed in rows andcolumns.

Known from the prior art are, in particular, sensor elements in seat matsensors essentially comprising two electrically separating filmsarranged in a mutually parallel manner. Between said two films isdisposed a likewise electrically separating interface layer whichmaintains the distance between the two films. Electrically conductivefaces are applied to the mutually facing sides of the films betweenwhich high-resistance material is arranged, preferably air.

When the seat mat sensor is subject to pressure, for example the bodyweight of a vehicle occupant, the conductive faces are brought intoconductive contact with each other, so that the electrical contactresistance between the two conductive faces is dependent on the pressureamplitude.

The conductive faces of known sensor elements, hereinafter also referredto as sensor cells, have rotational symmetry about the faceperpendicular. In a sensor arrangement in the form of a seat mat sensor,a plurality of similar circular sensor cells are arranged in a planarmanner on a vehicle seat.

Sensor arrangements of this kind are known from U.S. Pat. No. 5,896,090A, German Application No. 200 14 200 U1 and German Application No. 42 37072 C1; suitable sensor cells for these sensor arrangements aredisclosed in U.S. Pat. No. 4,314,228, German Application No. 200 14 200U1 and U.S. Pat. No. 4,314,227.

The problem with the known sensor arrangements is that the sensor cellsare mostly affixed to an uneven surface of a vehicle seat therebyexposing the sensor cells to differing tensile or bending loads,depending on their position on the seat mat sensor, in particular alongfold lines of the sensor arrangement.

The sensor elements known from the prior art are designed to respondvery sensitively to such tensile or bending loads on the fold lines.Consequently the signal characteristic of the sensor elements dependenton the weight acting upon them changes. The bending of the sensorelements usually displaces a constant offset PL of the signalcharacteristic to lower values as shown in FIG. 4. This signalcharacteristic displacement can have various repercussions for variousmeasurement systems designed for recognizing seat occupancy.

An occupant may be detected on the vehicle seat even if only a small,light object is located on it. In the event of vehicle impact this falseinformation would cause an airbag to inflate unnecessarily in order toprotect the supposed vehicle occupant.

In other systems designed for recognizing seat occupancy, a shift in theconstant offset of the sensor characteristic causes a false weight orfalse weight distribution of a vehicle occupant to be reported to theoccupant safety system. If an occupant safety device is, for example,released too early on the basis of this false information, this canresult in serious injuries to a vehicle occupant.

The change in the sensor characteristics when the sensor element isexposed to a tensile or bending load on a vehicle seat fold line isreferred to as the preload effect.

Owing to the preload effect an arrangement of sensor cells in seat matsensors is often avoided at heavily curved points or on seams of avehicle seat, although the lack of measurement at such points results ina loss of important information regarding the vehicle occupant. This canalso lead to serious injuries to a vehicle occupant in the event of anaccident involving impact if the occupant safety system was unable toprovide the optimum protection for the vehicle occupant through lack ofinformation.

A further measure for avoiding the preload effect is to reduce the sizeof the conductive faces of the known sensor cells.

With small sensor cells the constant offset (PS) of the sensorcharacteristic generally has a higher value than with larger sensorcells (FIG. 4). As a result, the preload effect manifests itself only atfar greater bending loads than in the case of large-face sensorelements. However, the sensor characteristic of a smaller sensor cell(CS) loses some of its resolution accuracy by comparison with thecharacteristic of a larger sensor cell (CL), since the slope of itssignal characteristic becomes steeper. At the same time the range ofvalues of its characteristic also becomes smaller (FIG. 4).

Here again as a result of too much imprecise information regarding thevehicle occupant, an optimal protection effect of an occupant protectionsystem adapted to seating occupancy fails to take place.

SUMMARY OF THE INVENTION

The invention discloses an improved seat mat sensor so that the sensorelements also enable informative weight measurements to be made oninstallation-dependent fold lines on a vehicle seat.

In one embodiment of the invention, at least one sensor element in thesensor arrangement is not of a rotationally symmetric design and has theadvantage that, arranged along a installation-dependent fold line, ithas a very small preload effect yet, independently of this, it achievesa very good resolution of the weight-dependent sensor signal within avery large range of values.

The rotationally symmetric structure of the active faces of the knownsensor elements must therefore be forgone with such installationdependent fold lines in favor instead of a non-rotationally symmetricactive face of the sensor elements. Circular active faces are known fromthe prior art. These can be replaced by, for example, oval active faces.Further embodiments are also possible, however.

Fold lines on a vehicle seat are, for example, curved surfaces on a legsupport or on both seat side supports or along seams in the seat cover.

The active face of a sensor element is understood to be the planarprojection of the sensor region which, within the useful range of thesensor characteristic—that is to say, before an essentially constantmaximum value of the sensor characteristic is attained, contributes tothe sensor characteristic in a manner dependent on weight and affectingthe signal (FIG. 4).

For improved electrical contacting, a circular sensor element may quitepossibly have indentations on its circumference which, for example, areextended as a supply lead via the seat mat to the evaluation electronicsof the seat mat sensor. The active face of the sensor element isnevertheless circular. In this sense, the term rotational symmetry isalso used.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below with reference to the exemplaryembodiments in the drawings, in which:

FIG. 1 shows the oval face of a sensor element 1 according to theinvention with an electrical supply lead 11 in a diagrammatic view.

FIG. 2 a shows a cross-section through a known rotationally symmetriclarge sensor element.

FIG. 2 b shows the cross-section through the sensor element shown inFIG. 2 a influenced by a bending load.

FIG. 3 a shows a cross-section through a known rotationally symmetricsmall sensor element.

FIG. 3 b shows the cross-section through the sensor element shown inFIG. 3 a influenced by a bending load.

FIG. 4 shows a signal characteristic Si of a known rotationallysymmetric sensor element with a large face CL and of a sensor cell witha small face CS.

FIGS. 5 a and 5 b show further exemplary embodiments of the geometricshape of the faces of a sensor element A according to the invention.

FIG. 6 shows: the circular face of a known sensor element S in planview; and

FIG. 7 shows a possible arrangement of circular and oval sensor elementson a vehicle seat.

The measured variable is hereafter considered to be the force on thesensor cells due to weight.

A sensor element with a circular or oval active face is hereafter alsoreferred to in abbreviated form as a circular or oval sensor element.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an oval sensor element 1 according to the invention. Theoval 1 drawn with a continuous line is the upper of the two conductivesensor element faces arranged one above the other. The line leading awayfrom this to one side is its electrical supply lead 11. The lower of thetwo conductive faces is not shown. The active face is not rotationallysymmetric. A longest line of intersection 12 through the sensor elementis drawn as a dashed and dotted line.

When the sensor element is bent about the longest line of intersection12 the sensor signal changes, while a force due to weight acts on thesensor element in an otherwise unchanged manner. The constant offset ofthe characteristic curve of the sensor element is displaced towardssmaller values.

When bending occurs about an axis 13 perpendicular thereto, thedisplacement of the sensor characteristic is, however, substantiallymore pronounced. The numerical value of the constant offset is evensmaller, that is to say, with a bending axis 13 the preload effect ismuch greater than in the case of the axis 12 perpendicular thereto.

An installation-dependent fold line of the sensor arrangement 1therefore lies preferably along the longest line of intersection 12.

Owing to installation tolerances in the motor vehicle, this mostfavorable sensor element fitting position cannot always be achieved inpractice, however. The length of the line of intersection through thesensor element along the fold line can vary by up to ±30% from thelongest line of intersection 12.

This bending sensitivity principle will now be explained with referenceto FIGS. 2 a to 3 b. FIG. 2 a shows a cross-section through a knownsensor element in a seat mat sensor arrangement. An upper sensor film 5of non-conductive material is arranged parallel to a lower sensor film 7of the same kind. The two films are kept mutually spaced apart by aspacer 9 made of likewise non-conductive material 10. On each of themutually facing sides, conductive faces 6 and 8 are applied to thefilms. Between the two conductive faces is arranged high-resistancematerial, in particular air.

FIG. 2 b shows the cross-section of the sensor element shown in FIG. 2a, the sensor element being bent about a fold line under the effect of aforce. Since the two conductive faces 6 and 8 have the same length alongthe cross-section but are kept apart by their spacers 9, using a defineddistance, the upper contact face 6 is more heavily curved than the lowercontact face 8. This results in the two conductive faces 6 and 8 comingvery close together at the point of their greatest curvature. If the twocontact faces continue to be bent in this region, electrical contact canoccur between the two conductive faces 6 and 8.

Since the two faces are positioned on an electrically differentpotential, a current dependent on the contact resistance flows betweenthe two conductive faces. The greater the contact face, the smaller thecontact resistance between them. The variation in the contact resistanceaccording to the contact face thus generated is also utilized in themeasurement of a weight resting thereon.

Deflection of the sensor element until electrical contacting takes placebetween the two conductive faces thus generates a false weight signal.This is referred to as a preload effect. The evaluation unit of the seatmat sensor cannot distinguish between this preload effect caused bybending of the sensor cell and a weight signal.

The sensor signal generated by a bending of the sensor element isevaluated in the same way as a weight resting thereon. A plurality ofmethods can be used to evaluate the sensor signals in such cases asfollows.

To maintain a constant flow of current between faces 6 and 8 constant,the voltage is varied according to the contact resistance between thefaces. The greater the contact face of the two conductive faces, thesmaller the contact resistance becomes. The more the sensor element isdeflected, the smaller is therefore the voltage required for theconstant flow of current.

Alternatively, the sensor signals can also be evaluated at a constantvoltage. The measure used for the weight resting on the faces is thenthe variable flow of current over the contact resistance of the twoconductive faces 6 and 8.

FIG. 3 a shows the cross-section of a sensor element of a structureidentical to that shown in FIGS. 2 a and 2 b. The planar extension ofthe conductive faces in a cross-sectional direction is much smaller,however.

FIG. 3 b shows the cross-section of the sensor element shown in FIG. 3 aunder the effect of the same bending about a fold line out of the planeof projection. At the points of greatest curvature the distance betweenthe two conductive faces 6 and 8 is greater than the comparable distancein FIG. 2 b.

For an electrical contact to be produced between the conductive faces 6and 8, the smaller sensor element in FIG. 3 b must be considerably moreheavily curved than the larger sensor element in FIG. 2 b. With theknown sensor cell structure a smaller sensor element face is thereforeless sensitive to the preload effect than a larger face. The signalresolution of the characteristic curve is also reduced as follows,however.

FIG. 4 shows a characteristic curve for a large sensor element CL andfor a small sensor element CS in a diagrammatic view. For the sake ofclarity the graph assumes that the sensor elements have ordinaryrotationally symmetric conductive faces. The sensor signal Si is plottedalong the X-axis. The Y-axis shows the force due to weight G acting onthe sensor cell.

The continuous line CL represents the characteristic curve for thelarge-face sensor element as follows. After attainment of a minimumforce due to weight PL is the upper conductive face 6 of the sensorelement pushed through to such an extent that electrical contact withthe lower conductive face 8 is achieved. Curve CL slopes continuouslyupwards as a result of the increasing force imposed by weight G. Thecontact face between the two conductive faces 6 and 8 becomes larger andlarger until the sensor signal approaches a constant value. Thereafterno additional sensor signal can be generated even if there is a furtherincrease in the force due to weight G.

The broken line CS shows the characteristic curve for the small-facesensor element in diagrammatic form as follows. More force due to weightPS has to be applied with this sensor element than with the large-facesensor element before a minimum signal is first emitted. Thereafter thischaracteristic curve also slopes continuously upwards with an increasein the force due to weight G until it attains a constant value whichdoes not increase further even with a further increase in the force dueto weight G. This constant maximum signal value is lower for the smallersensor element than for the larger sensor element.

The slope of the characteristic curve CS is, however, steeper than thecurve CL. The signal of the small sensor element CS responds much moresensitively to a variation in the force due to weight G than the signalof the larger sensor element CL. The resolution accuracy of the forcedue to weight acting on the sensor elements is reduced as a result. Itcan, however, be very important, particularly for the purpose ofclassifying a vehicle occupant according to weight, for the sensorcharacteristic to permit the greatest possible resolution of the forcedue to weight acting upon the sensor. A characteristic curve with aslight slope is therefore preferable to a steeper characteristic curve.

In the case of sensor elements having the known structure having twospaced-apart conductive faces, the object of the invention is thereforeachieved preferably by conductive faces of the kind that, when deformedabout a fold line, have the largest possible face along that line. Thatis to say, their line of intersection through the sensor face should beas long as possible along the fold line. As already mentioned above, theline of intersection of the sensor faces along the fold line,installation-dependent0, usually achieves a length that can vary by upto ±30% from the maximum line of intersection.

With this sensor element shape, the sensor signal is decisivelyinfluenced only where there is a much greater deflection than is thecase with the known rotationally symmetric elements. In the extensiondirection perpendicular to the fold line the face is preferablysmaller—that is to say, the line of intersection of the sensor facesalong the perpendicular to the fold line is shorter—in order to achievevery good signal resolution.

The oval sensor face 1 shown in FIG. 1 is an exemplary embodiment of theinvention.

FIGS. 5 a and 5 b show further embodiments of the invention. There arevarious advantageous sensor faces depending on the position of thesensor element fold lines as follows.

FIG. 5 a shows a rectangular sensor face 14, which is especiallysuitable for a fold line K path likewise shown. The oval sensor face 1from FIG. 1 could just as well be used here, however.

FIG. 5 b shows a boomerang-shaped sensor element 15 as a preferredembodiment of the sensor face in the region of two intersecting foldlines K1 and K2.

FIG. 7 shows a possible arrangement of circular and thereforerotationally symmetric sensor elements S1, S2, . . . SN on a vehicleseat Sz. On the fold lines K1 to K4 of the vehicle seat the shape of themostly circular sensor elements S to SN has been modified to the ovaland therefore asymmetric, that is to say not rotationally symmetric,shape of the sensor elements A1 to AN.

The invention is not restricted to the exemplary embodiments disclosed.Instead a plurality of modifications and variations is possible, andthese can differ according to the position in which a sensor element isto be fitted.

1. A sensor arrangements for recognition of seat occupancy in a motorvehicle, comprising: a plurality of pressure-sensitive sensor elements,which are arranged distributed in a planar manner, electricalperformance of each being dependent upon local value of a measuredvariable; and at least one not rotationally symmetric sensor element onan installation-dependent fold line of the sensor arrangement with alongest line of intersection through an active face of the sensorelement along the fold line.
 2. The sensor arrangement according toclaim 1, the active face of the sensor element being of an oval shape.3. The sensor arrangement according to claim 1, the sensor elementshaving two non-conductive films arranged one above the other, aconductive face on the upper film, a second conductive face on the lowerfilm, and a region between the conductive faces filled withnon-conductive material.
 4. The sensor arrangement according to claim 1,wherein the sensor elements are substantially rotationally symmetric. 5.The sensor arrangement according to claim 1, wherein sensor elements arearranged on installation-dependent fold lines of the sensor arrangementin an exclusively non rotationally symmetric manner.